Re: [HACKERS] MIT benchmarks pgsql multicore (up to 48)performance

On Wed, Oct 6, 2010 at 6:31 PM, Ivan Voras <ivoras(at)freebsd(dot)org> wrote:
> On 10/04/10 20:49, Josh Berkus wrote:
>
>>> The other major bottleneck they ran into was a kernel one: reading from
>>> the heap file requires a couple lseek operations, and Linux acquires a
>>> mutex on the inode to do that. The proper place to fix this is
>>> certainly in the kernel but it may be possible to work around in
>>> Postgres.
>>
>> Or we could complain to Kernel.org.  They've been fairly responsive in
>> the past.  Too bad this didn't get posted earlier; I just got back from
>> LinuxCon.
>>
>> So you know someone who can speak technically to this issue? I can put
>> them in touch with the Linux geeks in charge of that part of the kernel
>> code.
>
> Hmmm... lseek? As in "lseek() then read() or write()" idiom? It AFAIK
> cannot be fixed since you're modifying the global "strean position"
> variable and something has got to lock that.

Well, there are lock free algorithms using CAS, no?

> OTOH, pread() / pwrite() don't have to do that.

Hey, I didn't know about those. That sounds like it might be worth
investigating, though I confess I lack a 48-core machine on which to
measure the alleged benefit.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company